1. Field of the Invention
The present invention is related, generally, to solar cells. More particularly, the invention relates to a method of making solar cells such that the efficiency of the solar cell is increased. This is performed by creating one dimensional or two dimensional diffraction gratings on either the substrate or the solar cell material itself to diffract particular wavelengths of the incident radiation into the plane of the solar device such that the thickness of the solar cell is reduced and hence the distance the minority carriers have to traverse within the semiconductor material is reduced thereby increasing the collection probability of the minority carriers generated.
2. Description of the Related Art
Semiconductor based solar cells have been around for a while [1]. Initially these were based on silicon p-n homogeneous junctions and were primarily in use for space applications. More recently, solar cells based on III-V materials and various variations of silicon solar cells have been developed [2-3]. Pricing, availability of oil as well as the polluting effects of oil-based energy is promoting the use of alternate sources of energy such as solar cells. Solar cells are being made using homogenous p-n junctions as in the case of silicon-based solar cells as well as heterogeneous junctions as in the case of CdTe, GaAs, and Copper Indium Gallium Selenide based solar cells. Solar cells operate by using a semiconductor material with an energy gap between the conduction and valence bands. Electromagnetic radiation such as visible light has waves of various wavelengths in a continuum. As these waves enter a semiconductor material, photons with energy greater than the energy band-gap of the semiconductor material are absorbed and electrons from the valence band are excited into the conduction band leaving a hole behind in the valance band. Electrons and holes are collected at each end of the device by electrodes in order to generate electricity. While traversing the thickness of the cell some of these photon generated electrons and holes interact with the semiconductor material itself and are re-absorbed or lost. The design of a solar cell is typically a compromise between higher absorption, which increases with increasing thickness of the active material and higher collection of the minority carriers, which decreases, with increasing thickness of the active material [4]. Increasing the layer thickness is good for increasing absorption of the incident radiation. However, increasing the thickness of the layers also reduces the probability of the minority carriers created to be collected. Hence, based on the particular solar cell, an optimum thickness is chosen to maximize the efficiency of the cell.